Network Working Group J. Klensin, WG Chair
Request for Comments: 1651 MCI
Obsoletes: 1425 N. Freed, Editor
Category: Standards Track Innosoft
M. Rose
Dover Beach Consulting, Inc.
E. Stefferud
Network Management Associates, Inc.
D. Crocker
Silicon Graphics, Inc.
July 1994
SMTP Service Extensions
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1. Abstract
This memo defines a framework for extending the SMTP service by
defining a means whereby a server SMTP can inform a client SMTP as to
the service extensions it supports. Standard extensions to the SMTP
service are registered with the IANA. This framework does not
require modification of existing SMTP clients or servers unless the
features of the service extensions are to be requested or provided.
2. Introduction
The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
effective basis for the relay function of message transfer agents.
Although a decade old, SMTP has proven remarkably resilient.
Nevertheless, the need for a number of protocol extensions has become
evident. Rather than describing these extensions as separate and
haphazard entities, this document enhances SMTP in a straightforward
fashion that provides a framework in which all future extensions can
be built in a single consistent way.
3. Framework for SMTP Extensions
For the purpose of service extensions to SMTP, SMTP relays a mail
object containing an envelope and a content.
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RFC 1651 SMTP Service Extensions July 1994
(1) The SMTP envelope is straightforward, and is sent as a
series of SMTP protocol units: it consists of an
originator address (to which error reports should be
directed); a delivery mode (e.g., deliver to recipient
mailboxes); and, one or more recipient addresses.
(2) The SMTP content is sent in the SMTP DATA protocol unit
and has two parts: the headers and the body. The headers
form a collection of field/value pairs structured
according to STD 11, RFC 822 [2], whilst the body, if
structured, is defined according to MIME [3]. The content is
textual in nature, expressed using the US-ASCII repertoire (ANSI
X3.4-1986). Although extensions (such as MIME) may relax
this restriction for the content body, the content
headers are always encoded using the US-ASCII repertoire.
The algorithm defined in [4] is used to represent header
values outside the US-ASCII repertoire, whilst still
encoding them using the US-ASCII repertoire.
Although SMTP is widely and robustly deployed, some parts of the
Internet community might wish to extend the SMTP service. This memo
defines a means whereby both an extended SMTP client and server may
recognize each other as such and the server can inform the client as
to the service extensions that it supports.
It must be emphasized that any extension to the SMTP service should
not be considered lightly. SMTP's strength comes primarily from its
simplicity. Experience with many protocols has shown that:
protocols with few options tend towards ubiquity, whilst
protocols with many options tend towards obscurity.
This means that each and every extension, regardless of its benefits,
must be carefully scrutinized with respect to its implementation,
deployment, and interoperability costs. In many cases, the cost of
extending the SMTP service will likely outweigh the benefit.
Given this environment, the framework for the extensions described in
this memo consists of:
(1) a new SMTP command (section 4)
(2) a registry of SMTP service extensions (section 5)
(3) additional parameters to the SMTP MAIL FROM and RCPT TO
commands (section 6).
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RFC 1651 SMTP Service Extensions July 19944. The EHLO command
A client SMTP supporting SMTP service extensions should start an SMTP
session by issuing the EHLO command instead of the HELO command. If
the SMTP server supports the SMTP service extensions it will give a
successful response (see section 4.3), a failure response (see 4.4),
or an error response (4.5). If the SMTP server does not support any
SMTP service extensions it will generate an error response (see
section 4.5).
4.1. Changes to STD 10, RFC 821
STD 10, RFC 821 states that the first command in an SMTP session must
be the HELO command. This requirement is hereby amended to allow a
session to start with either EHLO or HELO.
4.2. Command syntax
The syntax for this command, using the ABNF notation of [2], is:
ehlo-cmd ::= "EHLO" SP domain CR LF
If successful, the server SMTP responds with code 250. On failure,
the server SMTP responds with code 550. On error, the server SMTP
responds with one of codes 500, 501, 502, 504, or 421.
This command is issued instead of the HELO command, and may be issued
at any time that a HELO command would be appropriate. That is, if
the EHLO command is issued, and a successful response is returned,
then a subsequent HELO or EHLO command will result in the server SMTP
replying with code 503. A client SMTP must not cache any information
returned if the EHLO command succeeds. That is, a client SMTP must
issue the EHLO command at the start of each SMTP session if
information about extended facilities is needed.
4.3. Successful response
If the server SMTP implements and is able to perform the EHLO
command, it will return code 250. This indicates that both the
server and client SMTP are in the initial state, that is, there is no
transaction in progress and all state tables and buffers are cleared.
Normally, this response will be a multiline reply. Each line of the
response contains a keyword and, optionally, one or more parameters.
The syntax for a positive response, using the ABNF notation of [2],
is:
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RFC 1651 SMTP Service Extensions July 1994
(4) any additional SMTP verbs associated with the extension
(additional verbs will usually be, but are not required
to be, the same as the EHLO keyword value);
(5) any new parameters the extension associates with the MAIL
FROM or RCPT TO verbs; and,
(6) how support for the extension affects the behavior of a
server and client SMTP.
In addition, any EHLO keyword value that starts with an upper or
lower case "X" refers to a local SMTP service extension, which is
used through bilateral, rather than standardized, agreement. Keywords
beginning with "X" may not be used in a registered service extension.
Any keyword values presented in the EHLO response that do not begin
with "X" must correspond to a standard or standards-track SMTP
service extension registered with IANA. A conforming server must not
offer non "X" prefixed keyword values that are not described in a
registered and standardized extension.
Additional verbs are bound by the same rules as EHLO keywords;
specifically, verbs begining with "X" are local extensions that may
not be standardized and verbs not beginning with "X" must always be
registered.
4.4. Failure response
If for some reason the server SMTP is unable to list the service
extensions it supports, it will return code 554.
In the case of a failure response, the client SMTP should issue
either the HELO or QUIT command.
4.5. Error responses from extended servers
If the server SMTP recognizes the EHLO command, but the command
argument is unacceptable, it will return code 501.
If the server SMTP recognizes, but does not implement, the EHLO
command, it will return code 502.
If the server SMTP determines that the SMTP service is no longer
available (e.g., due to imminent system shutdown), it will return
code 421.
In the case of any error response, the client SMTP should issue
either the HELO or QUIT command.
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RFC 1651 SMTP Service Extensions July 19944.6. Responses from servers without extensions
A server SMTP that conforms to RFC 821 but does not support the
extensions specified here will not recognize the EHLO command and
will consequently return code 500, as specified in RFC 821. The
server SMTP should stay in the same state after returning this code
(see section 4.1.1 of RFC 821). The client SMTP may then issue
either a HELO or a QUIT command.
4.7. Responses from improperly implemented servers
Some SMTP servers are known to disconnect the SMTP transmission
channel upon receipt of the EHLO command. The disconnect can occur
immediately or after sending a response. Such behavior violates
section 4.1.1 of RFC 821, which explicitly states that disconnection
should only occur after a QUIT command is issued.
Nevertheless, in order to achieve maxmimum interoperablity it is
suggested that extended SMTP clients using EHLO be coded to check for
server connection closure after EHLO is sent, either before or after
returning a reply. If this happens the client must decide if the
operation can be successfully completed without using any SMTP
extensions. If it can a new connection can be opened and the HELO
command can be used.
Other improperly-implemented servers will not accept a HELO command
after EHLO has been sent and rejected. In some cases, this problem
can be worked around by sending a RSET after the failure response to
EHLO, then sending the HELO. Clients that do this should be aware
that many implementations will return a failure code (e.g., 503 Bad
sequence of commands) in response to the RSET. This code can be
safely ignored.
5. Initial IANA Registry
The IANA's initial registry of SMTP service extensions consists of
these entries:
Service Ext EHLO Keyword Parameters Verb Added Behavior
------------- ------------ ---------- ---------- ------------------
Send SEND none SEND defined in RFC 821
Send or Mail SOML none SOML defined in RFC 821
Send and Mail SAML none SAML defined in RFC 821
Expand EXPN none EXPN defined in RFC 821
Help HELP none HELP defined in RFC 821
Turn TURN none TURN defined in RFC 821Klensin, Freed, Rose, Stefferud & Crocker [Page 6]

RFC 1651 SMTP Service Extensions July 1994
which correspond to those SMTP commands which are defined as optional
in [5]. (The mandatory SMTP commands, according to [5], are HELO,
MAIL, RCPT, DATA, RSET, VRFY, NOOP, and QUIT.)
6. MAIL FROM and RCPT TO Parameters
It is recognized that several of the extensions planned for SMTP will
make use of additional parameters associated with the MAIL FROM and
RCPT TO command. The syntax for these commands, again using the ABNF
notation of [2] as well as underlying definitions from [1], is:
esmtp-cmd ::= inner-esmtp-cmd [SP esmtp-parameters] CR LF
esmtp-parameters ::= esmtp-parameter *(SP esmtp-parameter)
esmtp-parameter ::= esmtp-keyword ["=" esmtp-value]
esmtp-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; syntax and values depend on esmtp-keyword
esmtp-value ::= 1*<any CHAR excluding "=", SP, and all
control characters (US-ASCII 0-31
inclusive)>
; The following commands are extended to
; accept extended parameters.
inner-esmtp-cmd ::= ("MAIL FROM:<" reverse-path ">") /
("RCPT TO:<" forward-path ">")
All esmtp-keyword values must be registered as part of the IANA
registration process described above. This definition only provides
the framework for future extension; no extended MAIL FROM or RCPT TO
parameters are defined by this RFC.
6.1. Error responses
If the server SMTP does not recognize or cannot implement one or more
of the parameters associated with a particular MAIL FROM or RCPT TO
command, it will return code 555.
If for some reason the server is temporarily unable to accomodate one
or more of the parameters associated with a MAIL FROM or RCPT TO
command, and if the definition of the specific parameter does not
mandate the use of another code, it should return code 455.
Errors specific to particular parameters and their values will be
specified in the parameter's defining RFC.
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RFC 1651 SMTP Service Extensions July 19947. Received: Header Field Annotation
SMTP servers are required to add an appropriate Received: field to
the headers of all messages they receive. A "with ESMTP" clause
should be added to this field when any SMTP service extensions are
used. "ESMTP" is hereby added to the list of standard protocol names
registered with IANA.
8. Usage Examples
(1) An interaction of the form:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250 dbc.mtview.ca.us says hello
...
indicates that the server SMTP implements only those SMTP
commands which are defined as mandatory in [5].
(2) In contrast, an interaction of the form:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250-EXPN
S: 250-HELP
S: 250-8BITMIME
S: 250-XONE
S: 250 XVRB
...
indicates that the server SMTP also implements the SMTP
EXPN and HELP commands, one standard service extension
(8BITMIME), and two non-standard service extensions (XONE
and XVRB).
(3) Finally, a server that does not support SMTP service
extensions would act as follows:
S: <wait for connection on TCP port 25>
C: <open connection to server>
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RFC 1651 SMTP Service Extensions July 1994
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 500 Command not recognized: EHLO
...
The 500 response indicates that the server SMTP does not
implement the extensions specified here. The client
would normally send a HELO command and proceed as
specified in RFC 821. See section 4.7 for additional
discussion.
9. Security Considerations
This RFC does not discuss security issues and is not believed to
raise any security issues not already endemic in electronic mail and
present in fully conforming implementations of RFC-821. It does
provide an announcement of server mail capabilities via the response
to the EHLO verb. However, all information provided by announcement
of any of the initial set of service extensions defined by this RFC
can be readily deduced by selective probing of the verbs required to
transport and deliver mail. The security implications of service
extensions described in other RFCs should be dealt with in those
RFCs.
10. Acknowledgements
This document represents a synthesis of the ideas of many people and
reactions to the ideas and proposals of others. Randall Atkinson,
Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas
and text sufficient to be considered co-authors. Other important
suggestions, text, or encouragement came from Harald Alvestrand, Jim
Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per
Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.
Miller, Dan Oscarsson, Julian Onions, Rayan Zachariassen, and the
contributions of the entire IETF SMTP Working Group. Of course, none
of the individuals are necessarily responsible for the combination of
ideas represented here. Indeed, in some cases, the response to a
particular criticism was to accept the problem identification but to
include an entirely different solution from the one originally
proposed.
11. References
[1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/Information Sciences Institute, August 1982.
[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
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